FitMyTire

Tire Size Comparison Calculator

Enter your current size and a new one to see every dimension that changes — and exactly how far off your speedometer will read.

✕ 3.5% diameter change — outside the ±3% safe rule
Overall diameter
+1.1"
31.61" → 32.71"
Width
+20mm
265 → 285mm
Sidewall
+0.55"
7.3" → 7.85"
Ground clearance
+0.55"
at the axle centre

Speedometer impact

With the new tire, here's your true speed when the dash reads:

Dash 30 mph
31
true mph
Dash 50 mph
51.7
true mph
Dash 65 mph
67.3
true mph
Dash 80 mph
82.8
true mph

Odometer reads low by 3.5% — every indicated 100 miles is really 103.5.

Shop 285/70R17 →

What actually changes when you change tire size

Three numbers move when you change a tire size, and they don't move together. Bumping the width (the first number) makes the tire wider but barely changes its height. Changing the aspect ratio (the middle number) changes sidewall height, which changes overall diameter twice over — once at the top, once at the bottom. Changing the wheel diameter (the last number) changes height inch-for-inch unless you compensate with a lower profile.

Overall diameter is the one that ripples into everything else: speedometer accuracy, odometer mileage, effective gearing, ground clearance, and ABS calibration. That's why the calculator leads with the diameter change and the ±3% safe-range verdict.

The 3% rule, explained

Tire and vehicle manufacturers generally treat a 3% overall-diameter change as the threshold below which you can swap sizes without recalibrating. At 3%, your speedometer is off by about 2 mph at 65 mph — noticeable but not dangerous. Beyond 3%, the error grows, gearing feel changes, and on many modern vehicles the traction and stability systems start working with bad wheel-speed data. The calculator marks any pair outside ±3% so you know when a recalibration or a different size is worth considering.

Planning a bigger setup on new wheels too? Check the rim fitment calculator to make sure the width fits the wheel before you commit to a size here.

Frequently asked questions

How much bigger can I go without affecting my speedometer?
The widely used safe limit is a 3% change in overall diameter. Within ±3%, the speedometer and odometer error stays small enough that most vehicles tolerate it without a recalibration, and gearing/ABS behavior is barely affected. This calculator flags whether your chosen pair is inside that window.
Will a bigger tire make my speedometer read high or low?
A taller tire travels farther per rotation, so you are actually going faster than the dash shows — the speedometer reads low. A shorter tire is the reverse. The calculator shows your true speed at several dash readings so you know the exact gap.
Does a taller tire hurt gas mileage?
It can, in two ways. A taller, heavier tire raises rotating mass and effectively lengthens your gearing, which can drop acceleration and low-end efficiency. But it also lowers engine RPM at highway speed, which can slightly help on the freeway. The net effect depends on how you drive; the diameter change shown here is the main driver.
What does the +/- ground clearance number mean?
Overall diameter changes are split evenly top and bottom, so half the diameter change shows up as ground clearance at the axle. A tire that is 1 inch larger in diameter raises the axle centerline by half an inch.